(a) Field of the Invention
The present invention relates to a liquid developer for use in electrostatic photography which developer is of the type composed of a toner dispersed in a carrier liquid having a high electric resistivity and a low dielectric constant.
(b) Description of the Prior Art
Liquid developers for use in electrostatic photography are generally prepared by dispersing a toner consisting of a coloring agent such as carbon black and a synthetic resin like acrylic resin, phenol-modified alkyd resin, rosin, synthetic rubber, etc. or a natural resin as the principal ingredients, with the addition of a polarity controlling agent such as lecithin, metallic soap, linseed oil, higher fatty acid, etc. in a carrier liquid consisting of a non-aqueous solvent having a high insulating property and a low dielectric constant such as an aliphatic hydrocarbonaceous solvent.
In the case of such conventional liquid developers, however, because the resin and/or the polarity controlling agent separate and diffuse in the carrier liquid with the passing of time, the polarity becomes indistinct. Further, because the pigments contained therein undergo intensive secondary agglomeration which renders it difficult to reduce them to primary particles even by a commercial particle size reducing means such as a ball mill, attriter, ultrasonic disperser, etc., they are exposed on the surfaces of the toner particles, which causes lack of uniformity in the polarity controlling property of the particles. As a result, the transferability is poor, the image density is low, and the uniformity of solid image area, the sharpness and the resolving power become insufficient.
Besides, because the fixability of the toner is poor, these developers are hard to fix particularly on a slick paper with high smoothness. Further, because of their being poor in dispersion stability, they are apt to give rise to agglomeration of the toner which would render it impossible to use them over a long period of time.